Cellulose as a structural material is extremely strong with a theoretical modulus of around 250 GPa or a specific tensile strength of about 5200 kN-m/kg or about 18 times that of titanium. However, most cellulose is naturally present in plant lignocellulosic biomass as a biocomposite made of cellulose, hemicelluloses, lignin, etc., with a hierarchical structure. (Wegner et al., A fundamental review of the relationships between nanotechnology and lignocellulosic biomass, in The Nanoscience and Technology of Renewable Biomaterials. 1st ed: John Wiley and Sons, p. 1-41 (2009).) Existing manufacturing technology has focused primarily on producing papermaking fibers as one of the largest utilizations of lignocellulose. Advanced separation techniques are required to effectively liberate cellulose from lignocellulosic biomass in the forms of nano-crystals and nanofibrils made of elemental crystals or elemental fibrils, respectively. These cellulose nanocrystals (CNC) or nanofibrils (CNF) have very special mechanical and optical properties and have been identified as a powerful building block for producing high-quality, durable, light weight, and cost-effective products for a variety of applications.
There are several approaches for CNC and CNF production from cellulose. The acid hydrolysis approach was developed in the 1940s and 1950s and remains a primary process for CNC production. (Battista O. A., Industry and Engineering Chemistry 42(3):502-507 (1950); Mukherjee et al., Biochimica et Biophysica Acta 10:499-511 (1953); Bondeson et al., Cellulose 13:171-180 (2006); Chen et al., Carbohydrate Polymers 76(4):607-615 (2009); Moran et al., Cellulose 15:149-159 (2008).) Hydrocholoric acid was used in early studies with typical acid concentrations between 2.5 N to 6.0 N. (Battista et al., Industry and Engineering Chemistry 42(3):502-507 (1950); Nickerson et al., Ind. Eng. Chem. 39:1507-1512 (1947).) Sulfuric acid was later used at a concentration of 2.5 N or 22 wt. % for a period of one hour or longer at boiling conditions to disintegrate cellulose. (Nickerson et al., Ind. Eng. Chem. 39:1507-1512 (1947).) Nickerson and Habrie found that the amount of cellulose hydrolyzed was less than 25% from glucose measurements even after 8 hours hydrolysis. RAnby found that after several hours of hydrolysis, cellulose sol composed of cellulosic whiskers were obtained through centrifugation of the unhydrolyzed cellulose at 1000 g for 15 min. (Rånby B. G., Discussions Faraday Soc. 11:158-164 (1951).) However the yield of cellulosic whiskers was only about 30-40%.
The use of very strong acid (over 50 wt. %) in the acid hydrolysis was first reported by Mukherjee and Woods. (Mukherjee et al., Biochimica et Biophysica Acta 10:499-511 (1953).) They were able to hydrolyze cellulose into colloidal nano-whiskers or CNCs with good dispersing properties when an acid concentration of approximately 65 wt. % was used. Hydrolysis durations were tens of hours at temperatures between 20 to 40° C. Their process conditions, however, resulted in a very low CNC yield of approximately 30%. Using a sulfuric acid concentration of approximately 64 wt. %, the reaction time could be reduced to a couple of hours or less at temperatures between 45-50° C. (Beck-Candanedo et al., Biomacromolecules 6:1048-1054 (2006); Bondeson et al., Cellulose 13:171-180 (2006); Chen et al., Carbohydrate Polymers 76(4):607-615 (2009); Dong et al., Cellulose 5:19-32 (1998); Hamad et al., The Canadian J. Chemical Engineering 88:392-402 (2010).) An acid hydrolysis process optimization study using a commercial microcrystalline cellulose as feedstock was conducted using a sulfuric acid concentration ranging from 44.1 to 64.8 wt. %, temperatures from 40 to 80° C., hydrolysis duration from 10 to 120 min. and cellulose consistency from 5 to 15%. (Bondeson et al., Cellulose 13:171-180 (2006).) Total cellulosic solids reported varied from 0 to 95%. Partial flow birefringence was observed from some of the resultant cellulosic solid suspensions with a relatively high concentration of sulfate groups. However, the resultant cellulosic solid from runs with yields higher than 47% were agglomerates rather than colloidal suspension of CNC. Separation of the agglomerates was not attempted to verify whether or not CNC was produced. The optimal condition for CNC production was found at an acid concentration of 63.5 wt. % and a temperature of 44° C. for 130 min. using a cellulose concentration of 10% with CNC yield of approximately 30%.
The extent of sulfation and/or the degree of polymerization (DP) have been indentified as indicators for the existence of CNCs. (Hamad et al., in Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass. Washington, D.C.: American Chemical Society, p. 301-321 (2011).) Hamad and Hu have conducted hydrolysis of a softwood Kraft pulp using sulfuric acid concentrations of 16, 40, and 64 wt. % at 45, 65, and 85° C. for periods between 5 to 25 min. (Hamad et al., The Canadian J. Chemical Engineering 88:392-402 (2010).) They found that the total cellulosic solid yield was a constant at approximately 90% for DP above 120, but reduced abruptly when DP was reduced below 120, suggesting the cellulose was significantly deploymerized and in the form of CNCs. It appeared that there was no CNC produced at the two low acid concentrations with substrate DP greater than 120 and a lack of sulfate groups. The resultant cellulosic solids were identified as partially hydrolyzed pulp without sulfate esters, suggesting that no CNC was produced. Attempts to separate the hydrolyzed pulp to verify the existence of CNCs were not reported. The sulfate groups were detected in the resultant cellulosic solids only at the sulfuric acid concentration of 64% (w/w) at 65° C. for periods of 5, 15, 25 min. or at 45° C. or higher for 25 min. The yields, however, were below 40%.
The production of CNC has conventionally been separated from CNF production. Strong acid hydrolysis, as described above, has been used for CNC production, while mechanical processing with and without chemical or enzymatic preprocessing has been employed for CNF production. Mechanical methods, such as shearing and homogenization, used to produce CNF are described in Alemdar et al., Bioresource Technology 99:1664-1671 (2008); Andresen et al., Cellulose 13:665-677 (2006); Iwamoto et al., Applied Physics A: Materials Science and Processing 89:461-466 (2007); Nakagaito et al., Applied Physics A: Materials Science and Processing 78:547-552 (2004).) A solely mechanical method to produce CNFs from plant biomass is very energy intensive. Although, chemical and enzymatic pretreatments can reduce this energy consumption.